The present invention relates to wireless integrated circuits, specifically to logarithmic amplifiers.
We are presently in the midst of a wireless revolution. Mobile phones, once a novelty referred to as car phones, have become ubiquitous. Wireless personal data assistants, local and wide area networks, and computer connections are now everyday pieces of business equipment. Data and voice telecommunications have changed the structure of the economy, and have changed the way people live. And now, a host of new products, enabled by the Bluetooth standard, are poised to enter a marketplace driven by the promise of a wireless Internet. The present invention provides important improvements to a key circuit used in wireless systems.
This circuit is a logarithmic amplifier. These amplifiers are useful in buffering signals, providing gain in the intermediate frequency (IF) path of both the transmitter and receiver, and in receive signal strength indicators.
Wireless devices typically transmit and receive data through the air on high frequency electromagnetic waveforms, though some systems, such as satellite dishes and pagers, simply receive, and others merely transmit. Data transmission is begun by encoding the data to be transmitted. In Bluetooth systems, encoded data typically has a rate of 1.5 MHz and is used to modulate a high frequency electromagnetic carrier signal. This carrier signal is in the 2.44 GHz range. The modulated carrier signal is then applied to an antenna for broadcasting. The broadcast signal is referred to as a radio frequency (RF) signal. Data reception involves receiving the RF signal on two antennas, and selecting the antenna with the stronger reception. The signal is then amplified, demodulated, filtered, and decoded.
In typical wireless systems, data is transmitted and received in packets having a specified structure and length. Each packet begins with a preamble, at least part of which contains no information. Part of the preamble is received using each antenna, and the receive signal strength for each antenna are compared. The antenna with the better signal is selected, and used for the remainder of the packet. For proper antenna selection, it is desirable that accurate measurements are made, and that they do not fluctuate with temperature, processing, and supply voltage.
Accordingly, embodiments of the present invention provide a logarithmic amplifier having a reduced sensitivity to power supply changes. Specifically, a common-mode feedback circuit is used to adjust the common-mode output voltage such that it tracks the supply voltage. In this way, cascoded output current sources are isolated from supply variations. Also included is an offset amplifier that enables multiple amplifiers to be cascaded without the use of intervening AC coupling capacitors. The design reduces the tolerance of the pole location of the high pass filter characteristic of the offset amplifier.
Embodiments of the present invention provide methods and apparatus of amplifying signals. One exemplary method includes receiving a variable power supply, generating a variable bias current, and applying the bias current to a load such that an average output voltage is generated. The method further includes receiving an input signal, generating a current proportional to the input signal, and subtracting the current from the variable bias current. As the variable power supply changes value by a first amount, the variable bias current is varied such that the average output voltage varies by the first amount.
A further exemplary embodiment of the present invention provides a circuit for amplifying signals. The circuit includes an input stage configured to convert an input signal to a first current, a current source stage configured to provide a bias current capable of being varied, an output stage configured to convert the bias current, less the first current, to an output voltage, and common-mode feedback circuit configured to receive a common-mode feedback voltage and the output voltage. The common-mode feedback circuit varies the bias current such that an average of the output voltage is equal to the common-mode feedback voltage.
Yet a further embodiment of the present invention provides a circuit for amplifying signals. This circuit includes a first device coupled between a first node and a first terminal of a resistor, having a control electrode coupled a first input terminal, a second device coupled between a second node and a second terminal of the resistor, having a control electrode coupled a second input terminal, a third device coupled between a first supply node and the first node, having a control electrode coupled to a common-mode feedback circuit, and a fourth device coupled between the first supply node and the second node, having a control electrode coupled to the common-mode feedback circuit. The circuit also includes a first output load coupled between the first node and the second supply node, and a second output load coupled between the second node and the second supply node. The first output load and second output load couple to the common-mode feedback circuit.
A better understanding of the nature and advantages of the present invention may be gained with reference to the following detailed description and the accompanying drawings.